1. Field of the Invention
This invention relates to image intensifiers, in particular, image intensifiers utilising a high operating voltage but energised from a low voltage power source by means of dc to dc converters.
2. Description of Related Art
Image intensifiers usually require high voltages, typically 10 kV or more to make them operate. To allow them to be used with batteries or other low-voltage power sources, it is common to use a power supply comprising a dc to dc converter which accepts a low input voltage, for example 2.5 to 3 volts, and delivers the higher voltage, typically 10 kV or more, required by the image intensifier. Because of size restraints it has been proposed to construct the converter in the form known as a `wrap around` power supply, that is to say, the converter circuitry surrounds and is potted in the same cylindrical case as the intensifier. Such an arrangement is illustrated in FIG. 1, and comprises a cylindrical case a `wrap around` converter power supply 2, and an intensifier 3. As well as being compact, this arrangement allows the lead lengths of high voltage-carrying conductors to be kept to a minimum, which enhances safety.
FIG. 2 shows in diagrammatic form the operation of a simple image intensifier. The intensifier 3 is energised by a power supply 10, which generally comprises a dc to dc converter including an intermediate ac stage. Light 4 from an image-producing source, (not shown), produces an image on the photo-cathode 8. Electrons emitted from the photo-cathode are accelerated along paths 9 in an electric field and are focused by means of focus electrodes 7 so as to produce an image on a phosphor 6. The image so produced is an amplified version of the image on the photo-cathode 8, and can be viewed by the eye of an observer or further optical apparatus such as a video camera, (not shown), at 5.
However, a number of problems have been encountered with this arrangement. The first is that the varying magnetic field from the inductive components of the converter will tend to modulate the paths of the electrons inside the intensifier. Hence electrons from any particular point on the photo-cathode will not always impinge on the corresponding point of the anode phosphor, resulting in an image with reduced sharpness.
The second problem arises when the intensifier is used in conjunction with some form of electronic imaging system, when interference from the intensifier power supply can cause objectionable interference in the TV picture.
Prior art solutions to these problems have involved magnetic and electrical screening, but these techniques can be expensive to implement, making assembly more difficult and adding significantly to the cost and size of the system.
The present invention has arisen in an attempt to eliminate or reduce the effects of the disadvantages of the prior art.